In the gas phase process for production of polyolefins such as polyethylene, a gaseous alkene (e.g., ethylene), hydrogen, optionally a co-monomer and other raw materials are converted to polyolefin product. Generally, gas phase reactors include a fluidized bed reactor, a compressor, and a cooler (heat exchanger). The reaction is maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. The reactor vessel is normally constructed of carbon steel and rated for operation at pressures up to about 31 bars (or about 3.1 MPa). Catalyst is injected into the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations to provide for a continuous polymerization process.
Operation of most reactor systems is critically dependent upon good mixing for uniform reactor conditions, heat removal, and effective catalyst performance. The process must be controllable, and capable of a high production rate. In general, the higher the operating temperature, the greater the capability to achieve high production rate. However, as the operating temperature approaches and exceeds the melting point of the polyolefin product, the particles of polyolefin become tacky and melt. The next event is an interplay of forces which results in particles agglomerating with adjacent particles. The net result is the formation of fused sheets of polymer along the vessel wall and along other parts of the reactor system. Progressive cycles in this process may eventually result in the growth of the sheet and its falling into the fluid bed. These sheets interrupt fluidization, circulation of gas and withdrawal of the product from the reactor, requiring a reactor shutdown for removal.
Similarly, accumulation of resin at the cooler results in a progressively decreasing cooling efficiency, which can eventually lead to elevated operating temperatures and the aforementioned problems. For reasons not completely understood, the use of certain catalysts has been found to create resin accumulations in the system, particularly at the cooler.
The selection of the catalyst also greatly affects the overall process. Among available catalysts for the polymerization of polyolefins are chromium containing catalysts such as chromium oxide based catalyst systems. Such catalysts have been used and are of importance, especially for the production of high density polyethylene. In particular, these catalyst are of special importance in, for example, blow molding, film, and pipe applications. (See, e.g., U.S. Pat. Nos. 4,739,015, 5,244,987, 5,385,991, 6,022,933, 6,429,269, 6,891,001, 6,936,666, and 6,989,344, and EP 0 927 724 B1. See also, U.S. Pat. Nos. 5,410,002, 5,844,054, 6,180,729, 6,664,352, U.S. Patent Application Publication Nos. 2005/0054790 and 2005/0267269, and Chinese Patent Application Nos. CN96111424.X and CN97106144.0.) Other background references include WO 2006/107373 and WO 2001/49751.
Accordingly, it would be desirable to provide for an improved polymerization process utilizing chromium containing catalysts and other materials such as, for example, oxygen and organo aluminum compounds.